Pathology is a field of medicine where medical professionals determine the presence, or absence of disease by methods that include the morphologic examination of individual cells that have been collected, fixed or air-dried, and then visualized by a stain that highlights features of both the nucleus and the cytoplasm. The collection of the cells often involves capturing a portion of a person's body fluid, placing the body fluid on a slide, and viewing the fluid on the slide using a microscope.
One of the most commonly performed pathologic studies is the CBC (the Complete Blood Count). To perform a CBC, a sample of blood is extracted from a patient and then the cells are counted by automated or manual methods. The CBC is commonly performed by using an instrument, based on the principal of flow cytometry, which customarily aspirates anticoagulated whole blood and divides it into several analysis streams. Using the flow cytometer a number of primary and derived measurements can be determined including: i) red blood cell (RBC) count, hemoglobin (Hb), hematocrit (Hct), red blood cell indices (mean corpuscular volume, MCV, mean corpuscular hemoglobin, MCH and mean corpuscular hemoglobin concentration MCHC), red blood cell distribution width, enumeration of other red blood cells including reticulocytes and nucleated red blood cells, and red blood cell morphology; ii) white blood cell (WBC) count and WBC “differential” count (enumeration of the different normal white blood cell types, including neutrophils, lymphocytes, eosinophils, basophils and monocytes, and the probable presence of other normal and abnormal types of WBC that are present in various disease conditions); iii) platelet count, platelet distribution widths and other features of platelets including morphological features; and iv) other abnormal cells or other unusual cells or cellular components that may be in circulating blood. In flow cytometers, red blood cell, WBC, and platelet morphological characterizations are typically made indirectly, based on light absorption and light scattering techniques and/or cytochemically based measurements. Some advanced flow cytometers calculate secondary and tertiary measurements from the primary measurements.
Flow based CBC instruments generally require extensive calibration and control, maintenance, and skilled operators, and they have substantial costs associated with acquisition, service, reagents, consumables and disposables. One significant problem with these systems in routine use is that a large proportion of blood specimens require further testing to complete the assessment of the morphologic components of the CBC. This involves placing a sample of blood on a slide, smearing the sample against the slide to form a wedge smear, and placing the slide under a microscope. This process is often done manually by skilled medical technologists, which increases the cost and time to receive results from the tests. The direct visualization of blood cells on a glass slide must be performed whenever the results of the automated test require further examination of the blood sample. For example, a “manual” differential count is performed by direct visualization of the cells by an experienced observer whenever nucleated immature RBCS are found or WBCs suspicious for infection, leukemias or other hematologic diseases are found.
The proportion of these specimens requiring further review generally ranges from 10% to 50%, depending on the laboratory policy, patient population and “flagging” criteria, with a median rate of around 27%. The most frequent reasons for retesting include the presence of increased or decreased number of WBCs, RBCs or platelets, abnormal cell types or cell morphology, clinical or other suspicion of viral or bacterial infections.
In addition to additional work involved in performing manual differential counts, this process has a number of additional technical limitations. These include distortions of cell morphology because of mechanical forces involved in smearing the cells onto the slide, and cells overlapping one another, which makes visualization of individual cell morphology difficult.
Cytopathology is a subspecialty of pathology where medical professionals determine the presence, or absence of disease by the morphologic examination of individual cells that have been collected, fixed or air-dried, and then stained by a unique stain that highlights features of both the nucleus and the cytoplasm.
Examples of cytologic examination include the assessment of cells collected from the uterine cervix (The Pap Test), evaluation of urine samples for bladder cancer, assessment of lung samples for the presence of cancer or inflammatory diseases, assessment of aspirates from potential tumor sites, or the evaluation of samples collected from effusions in body cavities.
Cells that are collected for cytologic examination may be directly smeared onto a glass microscope slide, they may be deposited onto the slide by centrifugation, they may be collected by a filtration method, or they may be concentrated by liquid-based cytology methods such as the ThinPrep or SurePath methods. These approaches used different types of preservative solutions that typically are alcohol-based, and attempt to deposit the cells to preserve cell morphology.
There are several limitations to current cytologic preparation methods. These include distortions of cell morphology because of mechanical forces involved in smearing or sedimenting the cells onto the slide. The deposition of cells onto the slide may result in cells overlapping one another, so that individual cell morphology cannot be visualized. Additional limitations associated with current cytologic preparation methods include, although are not limited to, the following: inhomogeneous sampling of a specimen due to differential rates of sedimenting cells onto a slide; the loss of cell clusters during certain types of preparation methods; the loss of small cells in methods that depend on density gradient methods of preparation; damage or nonspecific loss of inflammatory cells in centrifugation methods; and the inability to determine the absolute number of cell types in a sample, which may be important in determining the number of abnormal cells in a sample or in counting the number of inflammatory cells to determine the predominant type of inflammation that is in a sample.
Examples of these limitations are found with the Pap testing techniques that may not adequately display certain cell clusters because of smearing or filtration processes used to prepare the slide. The density gradient preparation method employed by the SurePath method may not capture small, buoyant cells that float atop the gradient. Centrifugation methods may result in the inconsistent loss of small lymphocytes, which can be problematic when an accurate differential count of inflammatory cells is required. Certain types of inflammatory lung diseases, such as idiopathic pulmonary fibrosis and sarcoidosis are characterized by unique profiles of inflammation that can only be useful diagnostically if an accurate enumeration of those cells can be determined.
A method of preparation of cytology samples that would not distort morphology, and that would result in a homogeneous and quantifiable number of cells being placed on the slide would overcome the limitations of current preparation methods.
Other examples involving body fluids include the detection of cells or cellular components that may be circulating in the peripheral blood. For example, cells from a non-hematological tumor may be found in the blood and could be detected by visual examination or automated examination of a slide. Special markers such as antibodies may be used to tag these cells. Other cellular components such as specific proteins may also be present in the blood either intracellularly or extracellularly and could be evaluated on the slide, typically by using certain markers to tag these slides. Certain inclusions in blood cells may also be detectable; for example parasites.